1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing apparatus which pressurizes and fixes a toner image transferred to a sheet onto the sheet, more specifically, relates to an image forming apparatus including a structure to correct (i.e., cure) curl of a sheet generated in the fixing apparatus.
2. Description of the Related Art
In general, in a case where an image is printed on a sheet with an image forming apparatus, the sheet to which a toner image is transferred by a transfer medium such as a photosensitive drum and an intermediate transfer belt is conveyed to a fixing apparatus at which the toner image is fixed, and then, is conveyed to a discharge portion.
By the way, there is a fixing apparatus including a pair of rollers including a fixing roller and a pressure roller which is arranged as being faced to the fixing roller in a manner of being pressed thereto. Here, there may be a case where a heat roller and a pressure roller are utilized or a heat belt and a pressure belt are utilized instead of the fixing roller and the pressure roller. In the following, description will be made on a case where the fixing roller and the pressure roller are utilized.
The sheet to which the toner image is transferred is pressed at a nip portion between the fixing roller and the pressure roller, so that the toner image is fixed. In this case, pressing between the fixing roller and the pressure roller is performed by pushing both ends of a pair of the rollers. Accordingly, both the rollers are mutually pressed strongly at both end part ranges. However, the pressing force becomes weak at the center part range. Consequently, there occurs difference in fixing capability of the toner image to the sheet between a sheet portion passing through the center part range and sheet portions passing through the both end part ranges of the pair of rollers.
In order to solve the problem, there is an image forming apparatus having a structure in which shock is eased at the time when a sheet is introduced to a fixing apparatus having a fixing roller and a pressure roller as enabling that both the rollers are mutually pressed with even pressure while axis lines of a pair of rollers are intersected with each other. Since distance between a photosensitive drum and a pressure fixing device is shortened in accordance with downsizing, the image forming apparatus is provided with a structure for avoiding a situation that a front end of a sheet is engaged to the fixing roller of the pressure fixing device in a state that a rear end of the sheet is located at the photosensitive drum (see Japanese Patent Laid-open No. 63-096684).
Here, with the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 63-96684, curl possibly occurs at the sheet to which a toner image is fixed as being pressed between the fixing roller and the pressure roller as entering to a nip portion of the rollers owing to intersection between the axis lines of the pair of rollers. In the following, an example of typical curl occurring in the fixing apparatus will be described with reference to FIG. 8.
That is, in a case where respective axis lines 50c, 51c of a fixing roller 50 and a pressure roller 51 are intersected at an intersection angle θ1 as illustrated in FIG. 8, a sheet S to which a toner image is fixed is apt to be as follows. For example, upper corner curl (U1, U2) is likely to occur as the front-right end section and rear-left end section being regulated to the fixing roller 50 side. Further, lower corner curl (D1, D2) is likely to occur as the front-left end section and rear-right end section being regulated to the pressure roller 51 side. In short, when both of the rollers 50, 51 of the fixing apparatus are intersected as described above, there appear sections without forming a nip portion at the ends in the longitudinal direction of the rollers 50, 51. Accordingly, it is considered that the curl is generated as a result of regulation against the sheet S to which the toner image is fixed at the front-right end section and rear-left end section thereof being the fixing roller side and the front-left end section and rear-right section being the pressure roller side.
The present invention provides an image forming apparatus capable of appropriately curing curl which is apt to be generated at a fixing-completed sheet by adding a simple structure even in a case where axis lines of a pair of rollers are intersected in a fixing apparatus which pressurizes and fixes a toner image.